1. Field of the Invention
The present invention relates to a unique elastomer material having particular utility for gaskets, O-rings, gland packing and similar fluid system sealing applications in highly sensitive facilities such as ultra-pure water and liquid chemical distribution systems.
2. Description of Related Art
Certain manufacturing processes and procedures in the pharmaceutical, integrated circuit and semiconductor production industries require ultra-pure water. Water flow stream contamination due to the presence of suspended particles, ions, metals, total organic carbon and surface roughness (microbe growth) is especially harmful to product quality.
Particles released from wetted polymer components in an ultra-pure water system may contact semiconductor wafers and cause unwanted surface contamination or disrupt photolithography process steps, thereby decreasing yields. Concern for Ionic contamination addresses the reactivity of such pure water. Leached extractives from the wetted system components may have a corrosive or etching effect on semiconductor devices during fabrication that causes failure of the semiconductor device. Evaporation of solutions containing ionics may leave surface residue on the semiconductor device having potential for altering the electrical properties of the device. Metallic contamination of semiconductor devices may also alter the respective electrical properties of a semiconductive device. Total Organic Carbon (TOC) contamination may effect silicon oxidation, etching uniformity and breakdown voltage of semiconductor device gate oxides. Surface roughness may influence microbial proliferation, provide an entrapment area for microcontamination build-up and/or promote shedding of the polymer itself within a distribution system.
Although the term ultra-pure, as applied to water, is imprecise, the semiconductor equipment and material trade association SEMI, has undertaken an effort to define the fluid in terms of a material property specification for materials to be used in wetted contact with an ultra-pure water generation, storage, distribution and transport facility. As presently stated, this specification for polymer material components such as polyvinylidenefluoride (PVDF) is published as SEMI F57-0301. Related standards from ASTM International include ASTM D4327, ASTM D4779 and ASTM D5904. Related ISO standards include ISO 1167 and ISO12162.
The following data tables, published under the SEMI F57-0301 standard, illustrate the extremes of material stability required of materials used to store and distribute ultra-pure water.
TABLE 1SURFACE EXTRACTABLE IONIC CONTAMINATIONStatic Value @ 85° C. ± 5° C.Elementfor 7 days (μg/m2)Bromide≦100Chloride≦3000Fluoride≦60,000Nitrate≦100Nitrite≦100Phosphate≦300Sulfate≦300
TABLE 2SURFACE EXTRACTABLE METALLIC CONTAMINATIONStatic Value @ 85° C. ± 5° C.Elementfor 7 days (μg/m2)Aluminum≦10Barium≦15Boron≦10Calcium≦30Chromium≦1Copper≦15Iron≦5Lead≦1Lithium≦2Magnesium≦5Manganese≦5Nickel≦1Potassium≦15Sodium≦15Strontium≦0.5Zinc≦10
TABLE 3SURFACE EXTRACTABLE TOTAL ORGANIC CARBON (TOC)CONTAMINATIONStatic Value @ 85° C. ± 5° C.Descriptionfor 7 days (μg/m2)TOC≦60,000
TABLE 4SURFACE ROUGHNESS REQUIREMENTSDESCRIPTIONRa max. ValueExtruded≦0.25 (≦10) μm (μin.)Injection Molded≦0.38 (≦15) μm (μin.)Machined≦0.62 (≦25) μm (μin.)
Table 5, following, is a typical proprietary standard for extractives leached from component materials proposed for ultra-pure water systems by a 48 hr. exposure to an extraction medium of 2% HNO2 at 68°.
TABLE 5EXTRACTABLE CONTAMINATION LIMITSEXTRACTION LIMITELEMENTng/cm2Aluminum2.2Boron0.091Bromine4Calcium11.00Chlorine21Chromium0.10Copper24.3Fluoride1320Iron3.10Lead1.8Lithium<0.0002Magnesium1.4Manganese<0.002Molybdenum0.061Nickel0.380Phosphorous<4Potassium10.00Sodium10.0Sulfur500Zinc4.00
Assembly component joints such as pipe flanges and tank covers for fluid confinement and distribution systems are traditionally sealed by compliant materials such as natural or synthetic rubber. One of the properties of a suitable sealant material is compliance, i.e. a capacity to flow intimately under moderate compressive stress into any voids and surface mismatches in the opposing seal faces of a joint structure.
Another essential property of joint sealants is a stable resilience at the stress displaced position, i.e. a memory of it's unstressed shape or position and maintenance of sufficient biasing force to return to the original shape when the displacement stress is removed.
Other obviously essential joint sealant properties include fluid impermeability and insolvency to the fluid that is confined by the sealant.
It is the combination of these properties, coupled with the highly reactive nature of ultra-pure water, that severely restricts the material options available for sealing joints in an ultra-pure water storage and distribution system.
Materials such as Teflon® have a limited stress memory. Although a Teflon® gasket that is substantially inert to the reactivity of ultra-pure water may have sufficient compliance to initially flow under compressive stress to seal a fluid joint, in time, the Teflon® gasket will dimensionally stabilize (set) at the initially sealed position. Hence, when other components of the system reconfigure due to vibration or temperature change, the Teflon® gasket has no capacity to accommodate the changes. The Teflon® gasket will release the joint seal and allow the confined fluid to escape the closed system.
In an effort to overcome the memory loss characteristic of Teflon®, the prior art has adopted a laminated gasket or seal structure wherein a natural or synthetic rubber core gasket is externally clad by a thin Teflon® overlay. The substantially inert Teflon® outer cladding is wetted directly by the ultra-pure water while the rubber core of the lamination maintains a bias against the Teflon® cladding that is essential to the sealed joint engagement. Although effective, this compound material construction is relatively expensive for a gasket or O-ring.
Although monolithic seals are fabricated from the perfluoroelastomer manufactured by E.I. DuPont de Nemours Co. and marketed under the tradename of Kalrez®, these products are perceived by some to be the source of ultra-pure water contamination in the form of leached carbon compounds, metallics, anion species, water permeation and particles released by mechanical working.
Another elastomeric material that has found some utility as a monolithic sealant in the ultra-pure water industry is Gelastic™. Gelastic™ is published to be the subject of U.S. Pat. No. 5,994,450 and is described by that patent as an A-B-A copolymer that is plasticized by mineral oil or a combination of mineral oil and resin. Distinctively, a Physical Data Sheet published for Gelastic™ to the material an elastic deflection of 2000% and a tensile strength of up to 1600 psi and a 0 compression set at ambient conditions and 500% compression. However, because the material softness is controlled by blended oils, the presence of such oil in the material matrix renders the medium unsuitable for many applications.
The provisions of SEMI bulletin F57-0301 ¶2.6.3 specifically designate high purity grades of perfluoroalkoxy (PFA), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) as preferred materials to be used in contact with ultra-pure water.
Polyvinylidene fluoride (PVDF) is a high-molecular-weight polymer of vinylidene fluoride frequently used in ultra-pure water systems as a rigid or semi-rigid machine component or as a fluid carrier conduit. It is a crystalline material with a melting point of 338° F. and a density of 1.78 g/cc. PVDF has outstanding resistance to most mineral and organic acids aliphatic and aromatic hydrocarbons, alcohols, halogenated solvents, and oxidizing environments. PVDF has greater strength, wear resistance, and creep resistance than PTFE, FEP, and PFA. It resists most chemicals and solvents, including oxidizers such as liquid bromine and bromine salt solutions. PVDF has good weathering resistance, and will not support combustion in air. It has a high dielectric constant (8 to 9) and a high loss factor relative to other fluoroplastics. Typical property values for polyvinylidene fluoride follow in Table 6.
TABLE 6TYPICAL PVDF PROPERTY VALUESDensity0.064 lb/in3Specific Gravity1.78Water Absorption, 24 hrs @ 73° F.<0.04%Tensile strength, Ultimate, @ 73° F.7,800 psiTensile Modulus, 1% sec. @ 73° F.348,000 psiElongation, Ultimate, @ 73° F.80%Flexural Strength, @ 73° F.10,750 psiFlexural Modulus Tangent, @ 73° F.333,500 psiCompressive Strength, @ 73° F.11,600 psiIzod Impact Strength, Notched, @ 73° F.3 ft-lb/inRockwell HardnessR100Deflection Temp. @ 66 psi300° F.@ 264 psi235° F.Melting Point352° F.Coefficient of LinearThermal Expansion7.1 × 105 in/in-° F.Thermal Conductivity1.32 Btu-in/hr-ft2-FFlammabilityV-OVolume Resistivity @ 73° F.5 × 1014 ohm-cmDielectric Constant9
The foamed derivative of rigid PVDF, ULTRAFLEX®, has many of the stable characteristics of PVDF desirable in an ultra-pure water system. Furthermore, because of its compliant character, ULTRAFLEX® has some of the characteristics of a desired ultra-pure water system sealant. However, the open-cell structure of ULTRAFLEX® is also highly permeable. High permeability is usually a rejection characteristic for sealant materials.
An object of the present invention, therefore, is a high purity, stable, and thermoformable material having suitable characteristics for a fluid system joint seal in an ultra-pure water system.
Another object of the invention is a consolidated PVDF material having elongation and compression set properties suitable for use in a fluid system joint seal.
Also an object of the invention is a procedure or process for treating foamed PDVF material to convert the foamed PDVF to a consolidated, pliable elastomer having a low compression set characteristic.